1. FIELD OF THE INVENTION
This invention relates to a small-size coreless dc vibrator motor for use in a wireless silent alerting device. The alerting device is vibrated by the vibrator motor upon receiving a radio call signal and transmits the vibration to the wearer of the device so that the wearer becomes aware that he is being called without being noticed by others and, therefore, without disturbing others.
2. DESCRIPTION OF THE PRIOR ART
FIG. 5 shows one of the conventional small-size vibrator motors. In FIG. 5, letter M denotes a cylindrical dc motor which has a rotatable output shaft denoted by letter S. Letter W denotes an eccentric weight and mounted on the shaft S. The motor vibrates as the shaft turns because of the eccentric and unbalanced mass distribution about the axis of the shaft S.
Such a conventional vibrator motor having an eccentric weight on the rotatable shaft, however, requires an extra length of the shaft on which the weight is mounted and an additional space for the weight to occupy for rotation. The weight is normally made of a high density metal, such as a tungsten-based alloy, so as to create a maximum unbalanced centrifugal force out of a very small size eccentric weight. Since such a high density metal for the eccentric weight is very costly, the overall production cost of the vibrator motor has to be substantially increased because of the cost of the weight.
Furthermore, this type of conventional vibrator motor normally employs a cylindrical permanent magnet and a pair of bearings for holding the rotatable shaft. The bearings are normally disposed diametrically inside the cylindrical magnet. In case the bearings are made of an oil-impregnated iron, the bearings themselves can serve as magnetic paths. However, if the bearings are made of an oil-impregnated bronze-based alloy, which is not magnetically conductive but is typical as a bearing material, an iron tube has to be additionally disposed between the bearings and the inside periphery of the permanent magnet in order to provide magnetic paths.
On the other hand, it is desired that the radial thickness of the cylindrical permanent magnet is as large as practically possible. However, in the case of the above described conventional motor, the radial thickness of the cylindrical magnet has to be considerably curtailed not only because of the presence of the rotatable shaft and a clearance immediately around the shaft but also because of the additional room necessary for accommodating the bearings and the iron tube, if any; all of these are normally present diametrically inside the cylindrical magnet. Whereas the outside diameter of the cylindrical permanent magnet must naturally be limited because of the inevitable limit of the diameter of the motor which has to be made as small as possible.
The loss of the radial thickness of the cylindrical magnet gives rise to a decrease in the permeance at the operating point because the ratio of the radial thickness of the cylindrical magnet to the amount of the air gap (i.e. the distance between the outside periphery of the cylindrical magnet and the internal periphery of the case of the motor, wherein a cylindrical coreless winding assembly is disposed) is decreased. Accordingly, the effective magnetic flux density across the air gap is minimized and the cylindrical coreless winding assembly disposed therein is subjected to a less amount of magnetic flux, thereby causing the amperage for a required torque to be increased.
Furthermore, because the eccentric bearing is mounted on one end of the shaft, the bearing closer to the eccentric weight is subjected to a greater amount of unbalanced centrifugal force created by the eccentric weight and this gives rise to an accelerated wear to one bearing as compared to the wear to the other bearing, thereby shortening the life of the motor.
U.S. Pat. Nos. 3,623,064, issued Nov. 23, 1971, and 3,911,416, issued Oct. 7, 1975, disclose small motors for wireless silent paging devices having add-on unbalanced weights mounted on the output shafts. U.S. Pat. No. 4,030,246, issued July 5, 1977, discloses a comparatively large industrial vibrator motor including a rotor shaft having a pair of fixed eccentric weights and a pair of eccentric weights which are angularly adjustable about the rotor axis, so that the amount of the vibration of the motor can be controlled.